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Thread: Delayed Choice Quantum Eraser

  1. #1 Is the idler entangled in any way after the signal is detected in the delayed choice quantum eraser. 
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    In the delayed choice quantum eraser experiment of Kim, once the signal has been detected and the idler continues, is the idler still entangled in any way?
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    I am thinking either no, or yes, it is entangled with an electron in the detector?
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  3. #3  
    KJW
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    Quote Originally Posted by plurabelle View Post
    In the delayed choice quantum eraser experiment of Kim, once the signal has been detected and the idler continues, is the idler still entangled in any way?
    No, a measurement of one member of an entangled pair destroys the entanglement between the pair.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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  4. #4 Is the delayed choice quantum eraser predicted by standard QM? 
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    Is the delayed aspect of the delayed choice quantum eraser experiment of Kim predicted by standard QM (in the way the outcome of the EPR experiment is)? Or does it tell us something new about the universe?
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  5. #5 Delayed Choice Quantum Eraser 
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    MODERATOR NOTICE: I've merged the two threads pertaining to the delayed choice quantum eraser into this one. Please confine discussion of this topic to this thread rather than starting new threads for different questions on the same topic.
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    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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  6. #6  
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    Quote Originally Posted by plurabelle View Post
    Is the delayed aspect of the delayed choice quantum eraser experiment of Kim predicted by standard QM (in the way the outcome of the EPR experiment is)? Or does it tell us something new about the universe?
    It's not clear to me the answer to your precise question but the delayed choice quantum eraser experiment involves no new physics beyond standard quantum mechanics. However, I will remark that the many-worlds interpretation of quantum mechanics provides what I consider to be the best explanation of quantum entanglement in general, with the delayed choice quantum eraser not providing anything inexplicable beyond the entanglement itself.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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    Does the delayed choice quantum eraser demand "spooky action over time"?

    I cannot (yet) see how, once the signal is measured and the idler is no longer entangled and then continues to the choice, there can be any relationship between the measurements (once coincidence correlated).

    [... I thought I might have had it "when measured the signal raises an electron which is then entangled with the idler", but that is nonsense as you say. ...]
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    Quote Originally Posted by plurabelle View Post
    Does the delayed choice quantum eraser demand "spooky action over time"?

    I cannot (yet) see how, once the signal is measured and the idler is no longer entangled and then continues to the choice, there can be any relationship between the measurements (once coincidence correlated).

    [... I thought I might have had it "when measured the signal raises an electron which is then entangled with the idler", but that is nonsense as you say. ...]
    The way I understand entanglement is to consider entire spacetimes in superposition. Each spacetime of the superposition is consistent in terms of the correlations of the particles that are entangled. It is the initial interaction that produced the entanglement but no further physical interaction occurs once the particles are separate. However, because it is entire spacetimes in superposition, the correlation between the particles exists over the entire spacetime, and the spatial or temporal separation of the particles is immaterial. And when a particle is measured, the entire spacetime is measured in the sense that all spacetimes that are inconsistent with the measured result are excluded from the superposition, and this includes those states of the entangled partner that are inconsistent with the measured result.
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    Perhaps I can turn my "question" around with the aim of highlighting my confusion.

    Why should the coincidence correlated measurements coincide, given that the idler is no longer entangled with the signal after the signal is measured? Why would different patterns be inconsistent?
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    Inconsistent in the logical sense: "If not, then one would be able to determine the position and momentum of this particle at this time in the experiment".
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  11. #11  
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    I'm guessing it has to do with causality.
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  12. #12  
    KJW
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    Quote Originally Posted by plurabelle View Post
    Why should the coincidence correlated measurements coincide, given that the idler is no longer entangled with the signal after the signal is measured? Why would different patterns be inconsistent?
    The cessation of entanglement by measurement doesn't pertain to the delayed choice eraser experiment because each particle is measured only once. If each particle were measured in two incompatible ways, then the first measurement will be correlated because this measurement was of an entangled state, but the second measurement will not be correlated because it was of an non-entangled state that is incompatible with the measurement (by "incompatible", it should be noted that if the slit the photon passed through is measured once or many times, the result will be the same each time).


    Two points are important to note:

    1: If the two particles are spacelike separated when measured, the notion of which measurement was first is invalid for relativistic reasons.

    2: Once the two particles have been separated, even though they are entangled, there is no physical connection between them that would cause something happening to one of the particles to affect the other particle.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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